The present invention relates to a method for producing a long ceramic body, and more specifically, to a method for producing a long, tubular ceramic body used, for example, as a heat transfer tube in a shell-and-tube-type heat exchanger and the like. One example of such a heat exchanger is described in U.S. Pat. No. 5,209,525, the entirety of which is incorporated herein by reference.
Heretofore, long ceramic bodies used as heat transfer tubes in ceramic shell-and-tube-type heat exchangers and the like have been manufactured by molding material powders using various molding methods into desired long shapes, calcining the molded bodies as needed, and then heating and firing the molded bodies while suspended in a sheath in spaced arrangement with the interior surface of the sheath. However, slight differences in density within the molded body tended to cause warping in the lower portion of the body during firing.
The method disclosed in U.S. Pat. No. 5,514,313 was developed to address the warping problem, and is depicted in FIGS. 2(a) and 2(b) (the sheath is not shown). According to the disclosed method, the long ceramic body 5 is first fired at a temperature at which sintering takes place until sintering is completed, as shown in FIG. 2(a). The body 5 is then inverted and fired again at a temperature at which sintering can take place, as shown in FIG. 2(b). When the body 5 is suspended during firing as shown in FIG. 2(a), warping is inhibited in the body upper portion A, because a large downward tensile force is applied to that portion by the weight of the body itself. However, warping occurs in the body lower portion B during the firing process, because a large downward tensile force, which would otherwise inhibit warping, is absent from that portion of the body. By inverting the body 5 so that the lower, warped portion B becomes the upper portion, as shown in FIG. 2(b), and then refiring the body at a temperature at which plastic deformation takes place, the lower portion B, which is now the upper portion during the second firing, is pulled downward by the weight of the body 5 and warping is corrected in portion B. The entire content of U.S. Pat. No. 5,514,313 is incorporated herein by reference.
In recent years, it has become desirable to increase the length of heat transfer tubes in the development of higher efficiency ceramic shell-and-tube-type heat exchangers. However, as the length of the tube increases, the amount of warp over the entire length of the tube also increases. It is preferred that the straightness of the tube over its entire length not exceed 0.5 mm in order to allow easy alignment and assembly of the tubes in the supporting structures of the heat exchanger. While the prior art method described can achieve straightness of 0.5 mm or less in relatively short (e.g., aspect ratio (L/W) of less than 50) heat exchanger tubes, it can achieve straightness only as low as 1.0 mm in relatively long (e.g., L/W 50) ceramic tubes. This is unacceptable as it gives rise to alignment and assembly problems during manufacture of the heat exchanger.
Accordingly, it is necessary to develop a method for producing long ceramic bodies (e.g., L/W&gt;50) with high shape accuracy, such as straightness, comparable to that of short ceramic bodies. Specifically, it would be desirable to achieve straightness of 0.5 mm or less along the entire length of a ceramic body having a length/width aspect ratio of at least 50.
Another problem with the conventional method described above is that the warp is corrected only by the weight of the suspended ceramic body itself. Accordingly, the degree to which the warp can be corrected varies from body to body. Moreover, it is difficult to form a group of long bodies all having the same shape, since the correction of warp in each individual body depends upon the physical characteristics of each body. That is, it is difficult to standardize the warp correction among all bodies in a group.